High-frequency discharge pumped axial-flow CO.sub.2 lasers are in the process of being developed because of their advantages of high-power output and stable oscillation. One example of such a laser is disclosed in Japanese Patent Application No. 61-243212 filed by the applicant.
A conventional axial-flow-type high-frequency discharge pumped laser oscillator device is illustrated in FIG. 3 of the accompanying drawings. The illustrated laser oscillator device includes a discharge tube 1 comprising four tube segments connected in series. However, the discharge tube 1 may have a desired number of tube segments dependent on the output to be produced. A total reflection mirror 2 and an output coupling mirror 3 are accurately positioned at the ends of the discharge tube 1. Denoted at 4 s an outgoing laser beam. The segments of the discharge tube 1 have gas inlet and outlet ports connected to a single roots blower 8. Cooling units 6, 7 serve to cool a laser gas heated by the discharge and the compression in the roots blower 8. The laser gas flows in the discharge tube 1 and gas delivery tubes in the directions of the arrows. Electrodes 9a, 9b.about.12a, 12b are connected to a high-frequency power supply 5. The gas flows in the discharge tube 1 at the speed of about 100 m/second. An electric discharge is produced in the discharge tube 1 by a high-frequency voltage applied by the high-frequency power supply 5 to generate laser oscillation.
FIG. 4 shows the structure of each discharge tube segment. The electrodes 9a, 9b are jointly of a double helical construction. The electric discharge is produced between the electrodes as indicated by the hatched area in the cross section of each discharge tube segment shown in FIG. 5. Tee plane of the electric discharge is twisted as shown in FIG. 6. This is to obtain a circular mode by successively twisting the non-circular cross sections of the discharges in the respective tube segments and averaging the successively twisted discharge cross sections.
Actual laser oscillator devices, however, fail to obtain a completely circular mode as shown in FIG. 7(a). As illustrated in FIG. 7(b), an actual mode is caused to deviate from a completely circular mode because the discharge gain in each discharge tube segment has a distribution in the direction of the gas flow.